This invention relates to aids to musical instrument tuning, particularly the tuning of keyboard instruments in the equal-tempered scale.
There are two broad categories of apparatus for tuning musical instruments: reference pitch generators and pitch comparators. A reference pitch generator is a device that produces sound of the correct pitch, such as a set of tuning forks or an electronically controlled frequency generator. A pitch generator, on the other hand is a device that usually provides a visual indication of the pitch of the note that is sounded. (The terms "pitch" and "frequency" are used interchangeably to denote that feature of the sound that is being tuned.)
Using a single tuning fork as a reference pitch generator has been traditionally the most common method of profession piano tuning. After several notes are tuned for a zero-beat with the tuning fork, the remaining notes are then tuned in a special sequence, employing a method know as interval tuning. This method requires considerable skill. It is also susceptible to the effects of cumulative error, which require frequent consistency checks and possible retracting of steps.
Using a large set of tuning forks (e.g., all the notes in one octave) reduces the cumulative error, but it still relies on very precise zero-beats being detected by ear. The same could be said of any reference pitch system, such as an electronically controlled frequency generator. However, a reference pitch generator is useful when tuning a note that is very far from the corrected pitch, such as when new strings are being installed on a piano.
Pitch comparators, on the other hand, can offer a much more accurate evaluation of the pitch of a sound. The best pitch comparators are actually phase comparators. They compare the phase of the sounded note against the phase of an internally generated signal of precise pitch. The rate of change of the phase difference between these two signals is a precise measure of the error in pitch.
The visual means employed to show the phase difference is usually a moving pattern. It is hard to see which way the pattern is moving if it is moving very fast, so phase-sensitive pitch comparators are only useful when the sounded note is too far off from the correct pitch.
In a pitch comparator, the internally generated reference frequency is not usually heard by the person using the device. It is only an electronic signal whose phase is compared against the phase of the sounded note. For example, in the motorized strobe tuners, the reference pitch, or frequency, exists as the rotation rate of the indicator. The sounded note, detected by a microphone, is used to modulate (strobe) a light which shines on the rotating indicator. The relative phase of these two signals is seen as the position of the visible strobe pattern. The rate of movement of the strobe pattern indicates the difference between the two frequencies. The goal is to tune the musical instrument until the visible strobe pattern is nearly stationary.
The motorized indicator in the motorized strobe tuner is bulky, hard to control, and prone to mechanical failure. (A solid state alternative is desirable.) One such device (described in U.S. Pat. No. 4,014,242, issued Mar. 29, 1977 for an "Apparatus for Use in the Tuning of Musical Instruments") uses quadrature reference signals and synchronous demodulation to display the phase comparison in a circle of LEDs. The brightness of each LED represents the degree to which the sounded note is in phase with that particular reference signal. The quadrature signals and their inverses provide a set of four reference signal that are supposed to cover all possible phase conditions. But since there are only four reference phases, poor phase resolution does not permit displaying sounded notes that are harmonically related to the reference note. So this device employs narrow bandpass filters that, together with the reference frequency octave selection, must be adjusted when different octaves are being sounded.
The frequency of the reference signal is the only source of error in a phase-sensitive pitch comparator. In order to achieve the highest accuracy, it is desirable to use precise digital frequency synthesis techniques to generate the reference signal. These techniques usually lock the reference signal to a quartz crystal oscillator using rational number frequency ratios. Unfortunately, the frequencies that comprise equal-tempered tuning are based on irrational number ratios that can only be approximated by rational numbers. To attain excellent accuracy, the whole numbers used in frequency synthesis must be very large. This means that either a very high frequency quartz frequency must be used, or else a very complicated series of whole-number multiplication and division circuits must be applied to the quartz frequency.
Usual frequency synthesis systems rely on fixed division ratios to achieve control of the generated frequency. In such systems, the period of the controlled frequency must be a whole-number multiple of the period of the high-frequency quartz reference signal. This limitation can be overcome by dynamically varying the division ratio so as to achieve a long-term average relationship between the quartz reference and the synthesized frequency. In musical instrument tuning, the average period of the reference signal is much more significant than the instantaneous period of each reference pulse.
Even after very good approximations to the perfect ratios are implemented, there is still the problem of offset adjustment. Usual frequency synthesis techniques do not adapt well to continuous adjustment of the ratios involved. The methods are better suited to "hopping" from one frequency to another. Therefore, most pitch comparators utilizing digital frequency synthesis, nevertheless use analog frequency synthesis to implement pitch scale offsetting. This is sometimes implemented by replacing the quartz oscillator by a variable frequency oscillator. A more accurate method is to use hetrodyning technology to mix a quartz signal with a (low frequency) variable signal. The resultant frequency sum retains much of the quartz signal's accuracy. However, the greater the offset range, the greater the potential frequency error.